Sleep apnea is a respiratory syndrome known to be present in about 8% of the adult male human population and 4% of the adult female human population.
The syndrome manifests itself as the repetitive cessation of, or large reduction in, breathing while the patient is asleep—respectively termed apneas and hypopneas. Apneas may be divided further into central apneas, where the cause of the apnea is the failure of the nervous system to activate the muscles responsible for respiration, and obstructive apneas, where the patient tries to breath but is prevented from doing so by the temporary collapse on inspiration of his or her upper airway. The reasons for such collapses are not completely understood but may include a loss of tone in those muscles which hold the airway open plus an anatomical disposition towards a narrow upper airway.
Prior to treatment the syndrome must be diagnosed. Conventionally, this is performed by an overnight study in a specialised sleep clinic, connecting the patient to electrophysical and respiratory measurement equipment to monitor physiological variables such as the electroencephalogram, blood oxygen saturation, heartrate, chest wall movement, and respiratory air flow during the various stages of sleep.
The attachment of the such monitoring equipment requires skilled staff and is often disruptive to the patient's sleep. Furthermore, the recording of all the physiological variables requires considerable computing power and the subsequent analysis, although assisted by computer, still requires considerable attention by the staff.
Monitoring of the patient's sleep in the patient's home traditionally uses a simplified form of the above-mentioned equipment which still may be complex and disruptive to the patient's sleep.
The measurement of less disruptive variables which correlate well with the traditional ones has been pursued as a way of making such sleep studies simpler to perform and less disruptive to the patient.
A device used in this area is the Static Charge Sensitive Bed (SCSB) described in U.S. Pat. No. 4,320,766 (Allihanka et al). U.S. Pat. No. 4,320,766 describes a mattress which outputs a single electrical signal that varies with the patient's movement. By suitable electrical filtering of the movement signal indications of body movement, respiration, snore and heartbeat are produced for subsequent display.
The SCSB principle was extended by Crawford and Kennard in their published UK patent application, GB 2 166 871 A (1984), for a Respiration Monitor. Here, strips of polyvinylidene fluoride (PVDF) were assembled in a common, parallel connection in order to give area coverage of a patient's respiratory movement. PVDF is a piezo-electric plastics material readily available in strips and sheets of minimal thickness.
A PVDF sensor has also been used in a device described by Siivola [Siivola J., (1989) New noninvasive piezoelectric transducer for recording of respiration, heart rate and body movements. Med. & Biol Eng. & Comput. 27, 423-424].
The clinical use of the SCSB is extensively described in the PhD thesis of Dr O. Polo (Dept of Physiology, University of Turku, Finland) republished as a supplement in Acta Physiologica Scandinavica Vol 145, Supplementum 606, 1992.
A PVDF film based device for detecting and recording snoring is also described in International Publication No. WO 96/36279 (Sullivan).
A limitation of the SCSB is that because of its inherent planar construction it cannot be used to localise the source of the movement it detects. Likewise, the above-mentioned devices also generate minimal spatial information. A major consequence of this is that the outputs of the said devices vary considerably with patient orientation. This limits the accuracy of information that can be derived from them.